The technology for building a fission bomb is quite advanced and complicated. The production of fissile material is only the first part. The physics and engineering of shaping the charges, building the detonators, the science of essentially slamming pieces of fissile material together via an explosion are the heart of nuclear weapons design. Iran is deficient in all of these areas, and will be for some time. Can they buy or invent the technology? Certainly. Can they “break out” and achieve a nuclear explosive device? Yes, at some point.
Were a future Iranian bomb to be detonated, it could be traced back to them. A ballistic missile or aircraft originating in Iran and arriving at the attack location can be tracked back. Nuclear explosions present signatures that can be assigned (or implied) to their manufacturer. The probability that Iran could wash their hands of any attack initiated by them converge on zero.
On the other hand, manufacture of an RDD is pretty basic. An RDD is essentially a conventional high explosive wrapped with radioactive material. The bomb detonates and the radioactive material is dispersed, contaminating the area plus dispersing downwind of the explosion site. The damage from the explosion, although it can be massive, is an incidental outcome. The purpose of an RDD is not destruction but area denial. Ultimately the purpose is panic/disruption of society: a particularly apt definition of “Terrorism” is “the Theater of the Obscene, for the Audience Watching” (credit goes to the FBI bomb techs who taught me this one). If you can make a bomb and have radioactive material, you can make an RDD. If you can explode an RDD, you can cause widespread horror and economic harm, and cause damage that can potentially last for generations.
Iran possesses the technology to develop and deploy an RDD today. Their nuclear centrifuges and reactors are taking ore primarily consisting of Uranium 238 (commonly found in nature but unusable in itself) and producing U-235 and U-234; they are also capable of producing Plutonium 239, which does not occur naturally. The output does not even have to be weapon-grade! It is easy to surmise that they can also produce non-trivial amounts of agents such as Polonium 210 (used in assassinations by poisoning the victim’s food – hence the tongue-in-cheek term, “a Polonium Sandwich”) and Cesium-137. Any of these isotopes can be employed in an RDD. They are extremely poisonous. Their half-lives range from 30 years (for C-137) to over 20,000 years (for U-234 and PU-239) to 700 million years (for U-235).
This is further complicated by the anonymity of delivery by proxy. Iran need not draw attention to themselves by delivering it themselves. They can sell or give RDD’s to Hamas, Hezbollah, al Shabab, al Aqsar, Islamic Jihad, al Qaida, AQAP… heck, they’d probably even sell them to ISIS if the price was right. The emergence of these shadow organizations has given rise to the term “Jihadistan”, a virtual country-without-borders circumscribed by ideology not geography. These groups have the means and motivation to smuggle an RDD into a country and set it off, even without regard to their own safety. Once it goes off, multiple actors would claim credit, and we might not have the intelligence to verify them or their source. The unique signature of an RDD, to my knowledge, is much less discernible than for a nuclear device (full disclosure, I do not have access to anything other than open source for these remarks. I hope we are better than that).
We have countermeasures for this scenario, but how good are they… really? We monitor radiation at most points of entry (where do we not? What are the lapses in technology or process for where we do?). We have NEST, Nuclear Emergency Search Teams, to respond (how quickly and effectively can they do so?). We have a military retaliation capability that’s a force of awe (are we as a country emotionally prepared to retaliate? Against whom? Jihadistan? With what? Our nukes?) This is not an opportunity for optimism.
As a former New Yorker, I remember the horror of 9/11 and the subsequent loss of innocence (and the loss of innocents). The good news was that the city did return to a normal, or at least a ‘new normal’, after a time. Imagine a dirty bomb contaminating downtown Manhattan, including Wall Street, and spreading cancer from Chinatown to Times Square. I cannot imagine a New York with a significant part of it carved out for the foreseeable future.
To paraphrase a famous movie quote, I am not afraid of the guy with 1,000 nuclear devices. I am scared of the guy with one. I am even more concerned with the guy who only has half of one.